Refactor `merge_popit_dicts` for Python 3 compatibility
Refactor `merge_popit_dicts` for Python 3 compatibility

Under py3, dict.keys is a set-like view; it can't be concatenated.

mysociety/yournextrepresentative,neavouli/yournextrepresentative,mysociety/yournextmp-popit,neavouli/yournextrepresentative,mysociety/yournextmp-popit,neavouli/yournextrepresentative,mysociety/yournextrepresentative,neavouli/yournextrepresentative,mysociety/yournextmp-popit,DemocracyClub/yournextrepresentative,DemocracyClub/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextrepresentative,neavouli/yournextrepresentative,mysociety/yournextmp-popit,DemocracyClub/yournextrepresentative,mysociety/yournextrepresentative,mysociety/yournextmp-popit